Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fractions as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. Doctor sweet means a mercaptan content in the product low enough to test "sweet" (as opposed to "sour") by the well known doctor test. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Sour hydrocarbon fractions containing more difficult to oxidize mercaptans are more effectively treated in contact with a metal chelate catalyst dispersed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The fraction is treated by contacting it with the supported metal chelate catalyst at oxidation conditions in the presence of a soluble alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.
The prior art shows that alkaline agents are necessary in order to catalytically oxidize mercaptans to disulfides. Thus, U.S. Pat. Nos. 3,108,081 and 4,156,641 disclose the use of alkali hydroxides especially sodium hydroxide for oxidizing mercaptans. Further, U.S. Pat. No. 4,913,802 discloses the use of ammonium hydroxide as the basic agent. The activity of the metal chelate systems can be improved by the use of quaternary ammonium compound as disclosed in U.S. Pat. Nos. 4,290,913 and 4,337,147.
Applicants have developed a catalyst and a process using the catalyst which is completely different from all the sweetening processes previously disclosed in the art. Applicants' process involves the use of a solid base instead of a liquid base. The solid bases which can be used to carry out the instant process are either a solid solution of metal oxides and/or layered double hydroxides (LDH). One example of a solid solution of metal oxides is magnesium oxide and aluminum oxide with varying Mg/Al ratios. An example of a layered double hydroxide is hydrotalcite which is a clay having the formula Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).multidot.4H.sub.2 O. Applicants have also found that these solid bases can be combined with a secondary component such as magnesium oxide to serve as the support for the desired metal chelate. In order to obtain appreciable conversion of mercaptans to disulfides applicants have further determined that an effective amount of a polar compound capable of serving as a proton-transfer medium must be added to the process. Examples of these compounds are water and methanol.
To applicants' knowledge there is only one report in the literature of a hydrotalcite material being used to oxidize mercaptans. Catalysis Letters, 11, pp. 55-62 (1991). This article describes the oxidation of 1-decanethiol in water. However, the catalyst and process described in this article are considerably different from applicants' process and catalyst. The reference discloses an LDH in which cobalt phthalocyanine is intercalated, between the LDH layers, whereas applicants' composition is a metal chelate, e.g., cobalt phthalocyanine dispersed on an LDH support. The reference uses a borate buffer to maintain the pH at 9.25 whereas applicants do not use any added base or buffer. Thus, the system disclosed in the reference and applicants' system are clearly different.
Finally, it should be pointed out that the prior art discloses that metal chelates can be dispersed on adsorbent supports such as clays or oxides (see e.g., U.S. Pat. No. 4,290,913). However, there is no indication that combinations of solid solutions or LDHs and magnesium oxide could be used as support nor that such catalysts would be able to function without an added liquid base.